1. Field of the Invention
The present invention relates generally to an actively controlled suspension system for an automotive vehicle, which controls suspension characteristics depending upon vehicle driving condition in order to suppress attitude change of a vehicular body. More specifically, the invention relates to a hydraulic circuit construction associated with an actively controlled suspension system which can prevent the suspension system from causing abrupt or sudden change of vehicular attitude upon ON-set of pressure source unit from inactive state.
2. Description of the Background Art
U.S. Pat. No. 4,702,490, issued on Oct. 27, 1987 which has been assigned to the common owner to the present invention, discloses one of typical construction of an actively controlled suspension system, in which a hydraulic cylinder defining a working chamber is disposed between a vehicular body and a suspension member rotatably supporting a vehicular wheel. The working chamber of the hydraulic cylinder is communicated with a hydraulic circuit including a pressurized working fluid source. A pressure control valve, such as an proportioning valve assembly, is disposed in the hydraulic circuit, which is connected to an electric or electronic control circuit to be controlled the valve position. The pressure control valve is controlled the valve position by a suspension control signal produced in the control circuit for adjusting pressure in the working chamber and whereby controlling suspension characteristics.
On the other hand, European Pat. Nos. 0 283 004, 0 285 153 and 0 284 053 discloses technologies for controlling the suspension systems constructed as set forth above, depending upon the vehicle driving condition for suppressing rolling and/or pitching of the vehicular body.
In one of the typical construction of the hydraulic circuit includes a pressure source unit which comprises a fluid pump drivingly associated with an automotive internal combustion engine so as to be driven by the engine output torque. This means that, when the engine is held inactive, the fluid pump cannot be driven for generating fluid pressure in the working fluid. Therefore, in response to turn OFF the engine, the hydraulic circuit becomes inactive to drop pressure to zero. This may cause variation of the fluid pressure in the working chamber of the hydraulic cylinder to cause lowering of the vehicular height. This is totally acceptable for degradation of the riding comfort and also for possibility of damaging of the vehicle body by causing contact with a step on the road.